REST API calls from the main application

While reviewing “PRO SPARK 4.5” project, I understand that MVC controllers are directly accessing the Repository layers. We have REST (Web API) in application, can we utilize API layer instead of directly accessing repositories?

Yes, if you have a rich API that supports the entire application functionality you can use the Web API.
The only thing to remember is that the performance will not be as good as with direct Repository calls.